Field of the Invention
This invention relates generally to the field of geological exploration for hydrocarbons. More specifically, the invention relates to a method of determining petrophysical properties of rock samples.
Background of the Invention
Traditionally, nuclear magnetic resonance (NMR) devices are designed to enclose a sample, producing a strong, homogeneous magnetic field that is used to analyze that sample for purposes including chemical analysis and anatomical imaging. The reverse (“inside-out” or “single-sided”) geometry, where the sample is located outside the NMR device, is technologically more challenging because of the difficulty in generating a homogeneous field over a large spatial region outside the magnet; as a result, NMR spectral linewidths tend to be too broad for chemical analysis, and images tend to be very blurry and have distortions. One area in which single-sided NMR has found application is oilfield borehole logging, where fluid typing is conducted within a small, shallow region in an underground rock formation in order to locate and characterize deposits of extractable hydrocarbon materials. Rather than the high magnetic fields (1 Tesla (T) or higher) used by most chemical and medical devices, logging tools employ “low-field” detection (generally below 1 T), with a field generated by one or more permanent magnets and detection occurring over a range of several inches into the formation wall. Fluids are characterized based on their diffusion and spin relaxation properties, a modality that is compatible with the field inhomogeneity endemic to the tool configuration. Recently, there has been increasing development on single-sided NMR devices for portable applications such as materials analysis. These tend to be designed for more general applicability than the specialized logging tools, although they operate on the same basic principles. Currently, there is no sensor available that provides a high-resolution porosity map of a subsurface rock sample, which will be critical for integrating the measured petrophysical properties and performing multi-scale analysis on the range of micro to field scale. A single-sided NMR sensor is the only solution for efficiently measuring a porosity map on the millimeter-to-centimeter scale.
Consequently, there is a need for improved sensors and methods to analyze rock and/or core samples from subsurface formations.